1. Field of the Invention
The present invention relates to a solder-coated printed circuit board having a solder layer of a thickness necessary for soldering electronic parts leads on a pad and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, to mount a surface mounting part on a printed circuit board, a solder paste is coated on a pad by means of printing, an electronic parts lead is placed on the solder paste, and the printed circuit board is heated to melt the solder paste, thereby soldering the surface mounting parts. Recently, however, as the array pitch of the parts leads is decreased, the array pitch of the pads is also decreased. When the array pitch of the pads is decreased to 0.5 mm or less, to print the solder paste becomes difficult, resulting in a frequent soldering defect such as a bridge.
As a method of mounting a fine pattern part, for example, a TAB on a printed circuit board, a so-called gang bonding method is known in which a solder layer is formed on a pad by electroplating or electroless plating, a parts lead is placed on the solder layer, and a heating jig or the like is urged against the parts lead, thereby performing soldering. This method, however, has a problem in formation of the solder layer. That is, to form a solder layer by electroplating, a current-carrying liner must be connected to each pad. It is cumbersome to connect and disconnect the current-carrying liner. To form a solder layer with a relatively large thickness, time-consuming plating must be performed. On the other hand, it is difficult to form a thick solder layer by electroless plating, and a solder layer of a thickness necessary for soldering a parts pad cannot be formed at a practical level by electroless plating.
As a method of forming a solder layer on a pad of a printed circuit board, a method is known which utilizes a substitution between, of solder alloy components, an organic acid salt (for example, a lead salt of an organic acid) of a metal having a smaller ionization tendency and a metal powder (for example, a tin powder) having a higher ionization tendency than that of the metal included above the organic acid salt (Published Unexamined Japanese Patent Application No. 1-157796). When a paste-like solder precipitating composition containing a lead salt of an organic acid and a tin powder as major components is solid-coated on a pad region on a printed circuit board and heated, a solder alloy is selectively precipitated on the pad by the substitution between the lead salt of the organic acid and the tin powder. This method utilizes the above phenomenon. According to this method, a solder layer can be formed without causing a bridge within a short period of time even if the array pitch of the pads is 0.5 mm or less.
However, since this method is to precipitate the solder on the pad by a substitution between the lead salt of the organic acid and the tin powder, the shape of the precipitated solder layer is largely influenced by the shape and dimension of the pad. For this reason, even when a solder layer having a thickness necessary for soldering a parts lead is to be formed on the pad, a large amount of the solder sometimes attaches to a side surface of the pad depending on the shape and dimension of the pad, thus making it difficult to increase the thickness of the solder layer.
In the case of a printed circuit board to be mounted with surface mounting parts of, for example, the SOP (Small Outline Package) type, the QFP (Quad Flat Package) type, and the PLCC (Plastic Leaded Chip Carrier) type which has a multiple of leads, a pad array having a multiple of pads corresponding to the parts leads is formed. When a solder layer of a thickness necessary for soldering the parts leads is to be formed on a pad array having a multiple of pads arranged at a fine pitch of 0.5 mm or less, the solder layers of the pads located at two ends of the pad array become thicker than the solder layer of the pads located at the intermediate position, thus making it difficult to form the solder layers of a uniform thickness on all the pads.
This is due to the following reason. That is, when the paste-like solder precipitating composition is solid-coated on the pad array to precipitate the solder on each pad, a step coverage is caused on the pads by the paste-like solder precipitating composition coated at a portion other than the pads. The step coverage amount is larger on pads at the two ends of the pad array than that on intermediate pads since the pad at each of the two ends of the pad array has an adjacent pad only at its one side. As a result, the thickness of the solder layer on pads at the two ends of the pad array is increased. In this manner, if the thickness of the solder layer varies in the same pad array, parts lead floating occurs during parts mounting. Then, to preliminarily fix a part on the printed circuit board becomes difficult, and an open defect in which a lead of part is not electrically contacted with a pad tends to occur.
In addition, in this method, since the paste-like solder composition is coated on the entire portion of the pad array, an economical efficiency is needed.
When a part is to be mounted on a solder-coated printed circuit board in which solder layers are pre-formed on its pads, a flux must be coated on each solder layer in order to impart good wettability of the solder to the solder surface for parts mounting. Since the flux contains an ionic substance, if the flux remains after parts soldering, the electrical insulating characteristic is degraded. For this reason, the flux must be cleaned off after parts soldering. Recently, however, use of a cleaning agent such as Freon (Tradename of DuPont de Nemours, E. I., Co.) is restricted because of environmental pollution, and an alternative to required demanded.